In recent years, significant improvements in cost effective electronic devices, together with the widespread availability of low cost powerful small computers has led to a great proliferation of sophisticated peripheral devices for use with such computers. One of the most popular such peripheral device is the modem for effecting data communications over band limited analog communications channels.
Since the introduction of the Smartmodem 300 modem from Hayes Microcomputer Products, Inc. in 1981, the overwhelming majority of modems sold for use with personal computers have included at least one microprocessor chip for controlling the modem and providing other functional enhancements. Today, higher speed data communications are popular and common, up to and including the 9600 bit per second full duplex voice grade channel communication scheme specified by CCITT Recommendation V.32. Additionally, other very sophisticated computer peripherals, such as laser printers and the like, often require significant computing power within the peripheral. This requirement leads to the inclusion of high speed microprocessors and significant amounts of high speed digital circuitry within such peripheral devices.
Naturally, as a result of the fundamental laws of physics, as expressed by Maxwell's Equations, the employment of such devices produces radio frequency interference. All electrical engineering students are familiar with the elementary results modeled by the Fourier transform which shows that sharp rising edges on a periodic waveform produce a noise spectrum having significant components at the harmonics of the signal's fundamental frequency. Naturally, the result of pushing digital circuitry to higher clock speeds is that the harmonic frequencies of such signals become higher and higher.
As sophisticated computer peripherals entered the home in connection with low cost powerful computer systems in the late 1970's, the Federal Communications Commission of the United States of America promulgated Subpart J of Part 15 of Title 47 of the Code of Federal Regulations. These FCC regulations placed stringent limits on the allowable level of radio frequency signals which could emanate into free space from computer peripheral devices, as well as limits on the radio frequency signals which could be fed back into the power line cord of such a device. In particular, the regulatory environment in the United States as a result of these rules is to place stringent requirements on "Class B" computing devices which are generally defined as those which one can reasonably expect to find associated with the home computer.
The rationale for promulgation of these regulations was based on an increasing number of complaints about radio frequency interference from computing devices, particularly interference in the bands occupied by VHF broadcast television in the United States.
Thus, after the promulgation of these rules, a significant percentage of the computer peripheral manufacturing community in the United States, particularly those concentrating on devices to be used with small computers, went through a rigorous and steep learning curve with respect to design techniques for minimizing radio frequency interference from such devices. The knowledge acquired from this learning curve has continued to grow. However, as problems are solved, new ones are created by the concurrent introduction of higher speed devices. For example, not too many years ago 4 megaHertz was a practical upper limit on master clock speed for a typical microprocessor. Today, it is not uncommon to find microprocessors operating at master clock frequencies in the range of 16 to 25 megaHertz. This naturally leads to exacerbated problems of free space radio frequency interference signals.
As is well known to those skilled in the art, the main sources of radio frequency interference emissions from computer peripherals devices are (a) cables entering and exiting the devices; and (b) slits and openings in the enclosures. While cables remain the principal source of problems, primarily because they function both as cable and an effective antenna for the undesired radio frequency signals, manufacturers have gone to some significant extremes to seal up openings and prevent the inadvertent creation of slit and edge antennas on equipment enclosures. In the realm of suppression of signals from cables, it is now common practice to provide schemes for plugs, receptacles, and cables in which a secure connection to chassis ground is provided to conductors within the plug/receptacle combination, which conductors are in turn connected to shielding on the cable. This helps to greatly attenuate the antenna effect from the cables.
In the arena of prevention of inadvertent creation of slit antennas, it has become common practice to secure receptacle carrying panels or plates to other grounded portions of the equipment enclosure. This can cause assembly to become problematic when it is desired to, for example, have receptacles which are soldered onto the circuit boards of such devices and can still pass through a receptacle panel. The receptacle panel is typically, but not necessarily, located at the rear of the peripheral. While this is effective for RFI suppression, it leads to some significant undesirable constraints on physical assembly of the devices. In most modern electronic assembly procedures, all of the components which are to be soldered to the circuit boards are assembled to the boards in a manufacturing process separate from creation of the equipment enclosure. Therefore, if receptacles soldered to the board are used, it is necessary to perform some rather complex mechanical assembly steps to secure grounded conductors of such receptacles to a receptacle panel during the process of final assembly. This sometimes takes the form of riveting plate elements onto the circuit board, as is the common practice with expansion slot peripheral devices such as serial port cards and internal modems.
If such a scheme is used on larger devices, it is often necessary to employ more nuts and bolts between the receptacle panel and the rest enclosure in order to provide a secure chassis ground to the panel and prevent the creation of slit antennas.
When this combination is assembled, it can lead to a configuration in which several connector devices must be physically disconnected from the receptacle plate of the enclosure before the circuit board can be removed.
As is known to those skilled in the art, it is highly desirable in assembling electronic equipment to be able to provide the entire collection of circuit boards and connectors (often referred to as "the guts" of the device in the crude vernacular) into a unitary subassembly which may then easily be dropped into the enclosure for final assembly. The more complex procedures, described hereinabove, with respect to assembly of computer peripheral devices, have largely resulted from the requirement of good radio frequency interference ("RFI") suppression.
Naturally, it is possible to provide receptacle plates on the rear panels of equipment enclosures with openings of a specified size behind which a plurality of connectors soldered to a circuit board may be placed. Assembly techniques are available which can, within reasonable limits, assure good land area contacts between portions of the receptacles which need grounding and grounded portions of the equipment enclosure near the holes through which the receptacles pass. This is an acceptable arrangement for suppressing RFI but leads to the constraint that the dies (in the case of stamped metal enclosure elements) or molds (in the case of molded plastic enclosures) must be dedicated to the particular plug/receptacle configuration on the circuit board. So long as products are limited to one geographic market, there is essentially no problem with this.
However, in the case of modems, it is desirable to be able to provide a modem product compatible with both the power and telephone networks in various foreign countries. Those skilled in the art will be familiar with the fact that standard computer and telephone connector devices vary geographically throughout the world. This is also true for power line frequencies and accepted standard physical arrangements for plugs for same. As a result of this, the creation of specific dedicated holes in the receptacle panel of the enclosure, as described above, often leads to the requirement that a different set of holes be provided when essentially the same product is to be marketed overseas. In the case of injection molded plastic enclosures, this can lead to the very expensive requirement for a new mold for the portion of the enclosure which includes the receptacle panel when virtually nothing else needs to be changed about the enclosure.
Therefore, there is a need in the art to provide a receptacle panel usable on a computer peripheral device which can be placed in a large opening on an equipment enclosure.
Naturally, under the older method of providing a complete subassembly of the electronics, including the receptacles, as a unit which may be dropped into an enclosure, the only requirement for the enclosure was a sufficiently large hole where the receptacle plate resided to allow the user access to same. Heretofore, reversion to this preferred form of assembly has not been practical for the following reasons. In a device of any appreciable size, such a hole for receiving the receptacle panel will normally include at least one fairly long dimension. Securely placing the subassembly in a position that abuts such a hole in a manner consistent with good RFI suppression techniques is difficult for the following reasons. As the electronic assembly becomes more complex, more and more mechanical elements are specified to a particular size with a certain range of tolerances. The more complex the device, the more tolerances have to be considered. The greater the number of tolerances, the greater the range of overall results which can be encountered if, for example, all of the tolerances deviate from the ideal value in the same sense.
The net result of this is the significant probability that a long narrow slit can be created between a receptacle panel so assembled and the large hole in an enclosure for receiving the panel. When the long dimension of the slit is on the order of half the wave length of a radio frequency signal emitted by the circuitry of the subassembly, the slit will act as a significant radiator of that frequency. In a complex device, if the apparatus is sized to assure that good grounding contact will take place between the receptacle plate and the periphery of the hole on the equipment enclosure, then it is likely that certain tolerance conditions, all of the same sense, may be encountered which will cause the electronic/receptacle plate subassembly not to fit properly in the enclosure.
Additionally, receptacle plates tend to be oriented vertically on a panel of such enclosures and it is common to drop an electronic subassembly into the enclosure vertically. This leads to difficulty in urging the assembly laterally toward the hole to assure that the receptacle plate makes good contact.
Therefore, in order to provide the advantages of the essentially prior art method of assembly encountered with attaching a separate receptacle plate as part of the electronic subassembly, there is a need for an innovative way of overcoming the problems outlined above. In particular, there is a need for providing a receptacle plate which will defeat the natural tendency to create an elongated slit antenna at the opening in an enclosure for receiving a receptacle panel. This must be created without creating the additional requirement of external rivets, bolts, screws, and the like to be passed through the enclosure and the receptacle plate. Additionally, any such arrangement must be able to deal with reasonably expected values of mechanical tolerances for the elements of the subassembly enclosure, and still not create RFI problems when the device is manufactured. To this end, it should be noted that manufacturability of an arrangement which will dependably meet the FCC requirements with respect to RFI suppression is a significant problem for modern electronics manufacturers.